Clipping through stl file of porous material
You will want to use a boolean modifier.  Start by adding a cube that overlaps with your object.  Select your object, go to the modifiers tab, and add a boolean modifier.  Set the target object to be your cube, and select 'difference' as your operation.  

Before modifier: 



After modifier:

In this screen shot I've set the cube draw type to wire so you can see what the modifier does.  You could simply move the cube to a different layer.